Defect inspection method and device for same

ABSTRACT

In defect scanning carried out in a process of manufacturing a semiconductor or the like, a light detection optical system comprising a plurality of photosensors is used for detecting scattered light reflected from a sample. The photosensors used for detecting the quantity of weak background scattered light include a photon counting type photosensor having few pixels whereas the photosensors used for detecting the quantity of strong background scattered light include a photon counting type photosensor having many pixels or an analog photosensor. In addition, nonlinearity caused by the use of the photon counting type photosensor as nonlinearity of detection strength of defect scattered light is corrected in order to correct a detection signal of the defect scattered light.

BACKGROUND

The present invention relates to a defect inspection method forinspecting an infinitesimal defect existing on the surface of a sample,identifying the type and dimensions of the defect, and outputting theresult, and relates to a defect inspection device provided for themethod.

In order to sustain and/or improve the yield of a product on amanufacturing line of a semiconductor substrate, the semiconductorsubstrate and the thin-film substrate or the like is inspected for adefect existing on the surface. Known documents describing conventionaldefect-inspection technologies include Japanese Patent ApplicationLaid-Open No. Hei9-304289 (used as PTL 1), Japanese Patent ApplicationLaid-Open No. 2006-201179 (used as PTL 2) and US Patent Application Ser.No. 2006/0256325 (used as PTL 3). In accordance with these technologies,in order to detect an infinitesimal defect existing on the surface of asample, illumination light is converged to dimensions of several tens ofmicrons and radiated to the surface. Then, scattered light from thedefect is surface. Then, scattered light from the defect is convergedand detected to inspect the surface for the defect having dimensionsranging from several tens of nanometers to at least several tens ofmicrons. A stage for holding the sample (the object of inspection) ismoved in a rotation and a parallel movement to scan the surface of thesample by making use of an illumination-light spot moving along aspiral-like path. In this way, the entire surface of the sample isinspected.

In addition, in accordance with the technologies described in PTLs 1 and2, components of the scattered light from a defect are detected. To bemore specific, the components are emitted in a large-angle direction anda small-angle direction. Then, the component ratio is used foridentifying the type of the defect.

In addition, in accordance with the technology described in PTL 2, thedimensions of a detected defect are computed on the basis of thestrength of scattered light from the defect.

In addition, in accordance with the technology described in PTL 3, inorder to reduce a thermal damage incurred by the sample, while theobject of inspection is being inspected, the power of the illuminationlight, the scanning velocity of the illumination-light spot or thedimensions of the illumination-light spot are controlled. To put itconcretely, the thermal damage incurred by the sample is assumed to havea magnitude determined by the product of the illumination power densityof the radiated light and the radiation time. Then, the power of theillumination light, the scanning velocity of the illumination-light spotor the dimensions of the illumination-light spot are changed inaccordance with radial position of the illumination spot on the samplebeing scanned so that the magnitude of the thermal damage does notexceed a fixed value.

CITATION LIST Patent Literature

PTL 1: Japanese Patent Application Laid-Open No. Hei9-304289

PTL 2: Japanese Patent Application Laid-Open No. 2006-201179

PTL 3: US Patent Application Ser. No. 2006/0256325

SUMMARY

The defect inspection carried out in a process to manufacture asemiconductor or the like imposes requirements described as follows. Thedefect inspection requires a capability of detecting an infinitesimaldefect. In addition, the defect inspection requires that the dimensionsof a detected defect be measured with a high degree of precision. Inaddition, the defect inspection requires that a sample be inspected in anon-destructive way. (That is to say, the defect inspection requiresthat a sample be inspected without changing the properties of thesample). In the case of the same sample serving as an object ofinspections, the defect inspection requires that the inspection of thesame sample always gives the same result of inspection (that is, thesame number of detected defects, the same locations, the same dimensionsand the same defect type). In addition, the defect inspection requires acapability of inspecting a number of samples within a fixed period oftime.

In the technologies described in PTLs 1 and 2, particularly in the caseof an infinitesimal defect with a dimension not greater than 10nanometers, scattered light generated from the defect is extremely weakso that a defect signal is buried in noises of the detector. As aresult, the defect cannot be detected. In order to expose the extremelyweak scattered light, a detector having a high gain is used in manycases. Normally, an electronic multiplier is utilized. If aphotomultiplier tube is used, however, the gain is increased andhigh-speed detection is carried out so that effects of dark currentnoises of the detector undesirably increase. In order to avoid thisproblem, illumination power is raised. In this case, however, theincrease of the temperature of the sample rises so that the sampleincurs a thermal damage. By the way, the increase of the temperature ofthe sample is caused by illumination light. As an alternative, in orderto avoid it, the scanning speed of the sample is reduced. However, areduced scanning speed decreases a sample area which can be inspectedwithin a certain period or the number of samples which can be inspectedwithin a certain period. The above description reveals that it isdifficult to detect an infinitesimal defect at a high speed whileavoiding thermal damages.

On the other hand, the technology described in PTL 3 is devised so thatthe amount of illumination light radiated to a unit area at anyarbitrary position on the sample is fixed. In general, however, theamount of illumination light detected from a defect is proportional tothe 6th power of the size of the defect. It is thus difficult toincrease the light quantity in order to merely comply with this. As aresult, the gain of the detector must be raised substantially.

There is known a photon counting method which is a method for detectingvery weak light. In general, by carrying out photon counting on veryweak light in order to count the number of photons detected from thelight, the SN ratio of the signal can be improved. Thus, a stable signalcan be obtained with a high degree of sensitivity and a high degree ofprecision. A known example of the photon counting method is described asfollows. This method counts the number of pulse-current generations ofpulse currents generated by incidence of photons to a photomultipliertube or an avalanche photodiode. When a plurality of photons arriveduring a short period of time, causing a plurality of pulse currents toflow, the number of pulse currents cannot be counted. Thus, the lightquantity cannot be measured with a high degree of precision so that thismethod cannot be applied to inspection of defects.

In addition, another known typical photon counting method is explainedas follows. The method employs an array sensor configured as a matrix ofnumerous avalanche photodiode pixels. Photons incident to each of thepixels generate pulse currents. Then, a sum of the pulse currents isfound. A detector used in the method is called an Si-PM (SiliconPhotomultiplier), a PPD (Pixeleted Photon Detector), an MPPC(Multi-Pixel Photon Counter) or the like. In accordance with ameasurement method making use of this detector, unlike the singlephotomultiplier tube described earlier and a photon counter operating ina Geiger mode using avalanche photodiodes, the light quantity can bemeasured even for a case in which a plurality of photons are incident ina short period of time. In this method, the magnitude of the detectionsignal is measured from pulse counts each representing the number ofpulses output by each of numerous detectors arranged into a matrix.Thus, in order to detect light quantities ranging from a small lightquantity to a large light quantity, it is necessary to form a number ofpixels in the detectors.

If a number of pixels are formed in this array sensor, however, thelight detection efficiency in general decreases undesirably, raising aproblem that the sensor is easily affected by shot noises. In this arraysensor, it is necessary to provide an insensitive area on a boundaryportion separating pixels from each other. Thus, since the pixel sizedecreases, the aperture ratio of the pixel is reduced. In an inspectionmethod whereby illumination light is converged on the object ofinspection and scattered light is detected in order to detect a defect,scattered light from the surface of the sample is detected. Thus, thereare variations of light generated by the sensors. That is to say, theshot noises become one noise element. In order to reduce the shotnoises, it is necessary to detect as many photons as possible. If theaperture ratio of the pixel decreases, however, the ratio of the shotnoises to the signal increases so that the inspection sensitivity isreduced. If the number of pixels is raised, on the other hand, acountermeasure against the shot noises is improved but the linearity ofthe signal detection becomes poor. In particular, if strong light isradiated to the surface of the sample, the amount of scattered lightfrom the surface of the sample also increases and the scattered lightbecomes background scattered light. Since many sensors output pulsesignals, photons incident to the pixel are no longer detected from asmall defect on the sample. That is to say, a phenomenon similar to thatcaused by a reduced sensor aperture ratio occurs, decreasing thesensitivity of the defect detection.

In order to solve the problems described above, the present inventionprovides a defect inspection device wherein a detection optical systemfor detecting scattered light reflected from a sample is configured tocomprise a plurality of detectors. In the defect inspection device, aphoton counting detector with few pixels is applied as a detector fordetecting a small background scattered light quantity. In addition, aphoton counting detector with many pixels is applied as a detector fordetecting a large background scattered light quantity or an analogdetector is applied. In addition, nonlinearity caused by applying thephoton counting detectors as the nonlinearity of the detection strengthof scattered light is corrected in order to correct the defect scatteredlight detection signal.

That is to say, in order to solve the problems described above, thedefect inspection device provided by the present invention is configuredto comprise: table unit on which a sample is mounted so that the samplecan be moved; laser light source unit for radiating laser light;detection optical system for converging light reflected by the sample towhich the laser light radiated by the laser light source unit isradiated; light detection unit for receiving the reflected lightconverged by the detection optical system and converting the light intoan electrical signal; signal processing unit for receiving the signaloutput by the light detection unit receiving the reflected light,processing the signal and detecting a defect on the sample; sizecomputation unit for computing the size of the defect detected by thesignal processing unit; and output unit for outputting a result ofprocessing carried out by the signal processing unit and the sizecomputation unit to a display screen, wherein: the light detection meanshas a plurality of photosensors including a photon counting detectorconfigured to comprise a plurality of pixels; and the size computationunit has a conversion section for correcting nonlinearity of the outputof the photon counting detector employed in the light detection unit,the size computing unit computing the size of the defect by processingoutput signals of the plurality of photosensors employed in the lightdetection means, the output signals including a signal which has beenoutput from the photon counting detector and has been correctednonlinearity by the conversion section.

In addition, in order to solve the problems described above, the defectinspection method provided by the present invention is implemented by:radiating laser light to a sample mounted on a table moving in aparallel movement while rotating; converging light reflected by thesample to which the laser light is radiated; driving light detectionunit to receive the converged light reflected by the sample and convertthe light into an electrical signal; receiving the signal output by thelight detection unit receiving the reflected light, processing thesignal and detecting a defect on the sample; computing the size of thedetected defect by processing the signal output by the light detectionunit; and outputting information on the detected defect and informationon the computed size of the detected defect to a display screen,wherein: the light detection unit has a plurality of photosensorsincluding a photon counting detector configured to comprise a pluralityof pixels whereas the photosensors each receive light reflected by thesample and convert the light into an electrical signal; and the size ofthe defect is computed by correcting nonlinearity of output from thephoton counting detector, among signals output by the plurality ofphotosensors receiving light reflected by the sample, and processingeach output signals of the plurality of photosensors, the output signalsincluding an output signal of the photon counting detector whosenonlinearity has been corrected.

In accordance with the present invention, it is possible to detect botha small defect generating only very weak light and a deformed defectgenerating scattered light only at the backward.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing an entire outline configuration of adefect inspection device according to an embodiment of the presentinvention;

FIG. 2A is a sample top-view diagram showing an illumination area on asample;

FIG. 2B is a sample top-view diagram showing an illumination locus on asample;

FIG. 3A is a front-view diagram showing the front view of a detectionsection having a configuration making use of an analog detection sensor;

FIG. 3B is a front-view diagram showing the front view of a detectionsection having a configuration including a reflection optical systemmaking use of an elliptical mirror;

FIG. 3C is a front-view diagram showing the front view of a detectionsection having a configuration in which scattered light coming from aplurality of directions is converged to create an image on an imagesensor;

FIG. 3D is a front-view diagram showing the front view of a detectionsection having a configuration making use of a reflection optical systembased on a Schwarzschild optical system;

FIG. 4A is a celestial-sphere diagram showing an angle range in whichscattered light from a sample is detected on the surface of thecelestial sphere and a projection diagram showing the celestial-spherediagram projected on a plane;

FIG. 4B is a sphere-surface front-view diagram showing a typicallocation of a detection section on the surface of the celestial sphere;

FIG. 4C is a sphere-surface front-view diagram showing another typicallocation of a detection section on the surface of the celestial sphere;

FIG. 5A is a front-view diagram showing the front view of a regularreflection light detection optical system having a configuration foroptically blocking regularly reflected light by making use of a spatialfilter in order to detect only scattered light in proximity to theregularly reflected light;

FIG. 5B a front-view diagram showing the front view of a regularreflection light detection optical system configured to include adetection system in accordance with the Schlieren method;

FIG. 5C is a front-view diagram showing the front view of a regularreflection light detection optical system having a configuration forcarrying out ellipsometry processing for light regularly reflected bythe surface of the sample;

FIG. 6A showing the shape of an illumination spot on the surface of asample 1 and graphs representing detection-signal profiles in S1 and S2directions inside the illumination spot;

FIG. 6B is a front-view diagram showing the front view of the shape of 3illumination spots on the surface of a sample 1 in a state wherein thehalf portions of all the illumination spots overlap each other;

FIG. 6C is a graph representing a typical signal for a case in which thesame defect has been detected a plurality of times;

FIG. 6D is a graph representing the profile of a detection signal inwhich a device function has been convolved in an original signal;

FIG. 6E is a graph representing a profile obtained as a result ofdeconvolution by a device function for the profile of a detectionsignal;

FIG. 7A is a diagram showing a typical configuration of a detectionsection for a case in which a photon counting sensor is used and is alsoa front-view diagram showing the front view of the detection sectionhaving a configuration in which the photon counting sensor is placed ata location shifted from a light convergence point of a light convergenceoptical system;

FIG. 7B is a diagram showing a typical configuration of a detectionsection for a case in which a photon counting sensor is used and is alsoa front-view diagram showing the front view of the detection sectionhaving a configuration in which a non-spherical surface lens is placedat a location shifted from a light convergence point of a lightconvergence optical system and the photon counting sensor is placed at astage following the lens;

FIG. 7C is a diagram showing a typical configuration of a detectionsection for a case in which a photon counting sensor is used and is alsoa front-view diagram showing the front view of the detection sectionhaving a configuration in which a plurality of prisms are placed at alocation shifted from a light convergence point of a light convergenceoptical system and the photon counting sensor is placed at a stagefollowing the prisms;

FIG. 8 is a front-view diagram showing the configuration of a photoncounting type photosensor;

FIG. 9 is a top-view diagram showing the surface of a sample and servingas a diagram to be referred to in explanation of the polarizationcharacteristic of a scattered light distribution from the surface of thesample;

FIG. 10 shows graphs to be referred to in explanation of relationsbetween the incident-light quantity of a photon counting typephotosensor and the output of the detector;

FIG. 11 is a front-view diagram showing the front view of a detectionoptical system having a configuration which makes use of 2 photoncounting type photosensors and can be adapted to also an applicationwith a large scattered light quantity;

FIG. 12 is a circuit block diagram showing a signal processing sectionprovided by the present invention;

FIG. 13 is a front-view diagram showing the front view of a photoncounting type photosensor and serving as a diagram to be referred to inexplanation of an illumination state of light reflected to the photoncounting type photosensor;

FIG. 14 is a top-view diagram showing the top view of a sample in astate of being mounted to serve as a calibration sample for correctingnonlinearity of the relation between the incident-light quantity of aphoton counting type photosensor and the output of the detector;

FIG. 15 shows a graph representing an input/output characteristic of adetector obtained by making use of a calibration sample for correctingnonlinearity of the relation between the incident-light quantity of aphoton counting type photosensor and the output of the detector;

FIG. 16 is a flowchart showing a nonlinearity measurement sequenceexecuted by making use of a calibration sample for correctingnonlinearity of the relation between the incident-light quantity of aphoton counting type photosensor and the output of the detector;

FIG. 17 is a flowchart showing a flow of processing carried out tocompute the quantity of light incident to each detector, find a totallight quantity of light scattered by a defect and calculate thedimensions of the defect; and

FIG. 18 is a flowchart showing a flow of processing carried out tocompute the quantity of scattered light by converting output values of aphoton counting type photosensor on the basis of a model data vectorwithout directly converting the output values.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention are explained by referring to thediagrams as follows.

The configuration of an embodiment of the present invention is explainedby referring to FIG. 1. The embodiment is configured by installing anillumination section 101, a detection section 102 (102 a, 102 b and 102c), a stage 103 on which a sample 1 can be mounted, a signal processingsection 105, an entirety control section 53, a display section 54 and aninput section 55. The signal processing section 105 has an analogprocessing section 50, a defect determination section 51 and adefect-size-inference/defect-classification 52. If necessary, aregular-reflection light detection section 104 is provided forinspecting a defect having a large area or measuring the surface of thesample.

The illumination section 101 is configured to arbitrarily comprise alaser light source 2, an attenuator 3, a polarization device 4, a beamexpander 7, an illumination distribution control device 5, a reflectionmirror m and a convergence lens 6. A laser beam radiated from the laserlight source 2 is adjusted by the attenuator 3 to a desired beamstrength, adjusted by the polarization device 4 to a desiredpolarization state, adjusted by the beam expander 7 to a desired beamdiameter and radiated to an inspection-object area of the sample 1 byway of the reflection mirror m and the convergence lens 6. Theillumination distribution control device 5 is used for controlling anillumination strength distribution on the sample 1. FIG. 1 shows aconfiguration in which the illumination section 101 radiates light froman inclined position in a direction inclined with respect to the normalline of the sample 1. However, it is also possible to adopt aconfiguration in which light is radiated in a direction perpendicular tothe surface of the sample 1. It is also possible to make use of switchmeans for changing the illumination optical path from that of the formerconfiguration to that of the latter configuration and vice versa.

To detect a small defect close to the surface of the sample, as thelaser light source 2, it is possible to employ a light source whichoscillates a laser beam and has a high output of at least 1 W. The laserbeam has a wavelength which is hardly capable of penetrating into theinside of the sample. Examples of such a laser beam are an ultravioletlaser beam having a short wavelength and a vacuum ultraviolet laserbeam. To detect a defect in the sample, on the other hand, as the laserlight source 2, it is possible to employ a light source which oscillatesa laser beam having a wavelength capable of easily penetrating into theinside of the sample. Examples of such a laser beam are a visible laserbeam and an infrared laser beam. In accordance with requirements, it ispossible to arbitrarily select an inclined-direction illumination lightsource or a perpendicular-direction illumination light source.

The stage 103 comprises a translation stage 11, a rotation stage 10 anda Z stage (not shown in the figure). FIG. 2A shows an illumination area(illumination spot 20) on the sample 1. FIG. 2B shows a relation with ascanning direction of movements made by the rotation stage 10 and thetranslation stage 11 and shows a locus drawn on the surface of thesample 1 due to the movements as a locus of the illumination spot 20.FIG. 2A shows the shape of the illumination spot 20. The shape is anelliptical shape formed with the long axis oriented in a direction S1and the short axis oriented in a direction perpendicular to thedirection S1 as a result of illumination distribution control executedby the illumination section 101 or illumination from the inclineddirection. As shown in FIG. 2B, due to a rotational movement made by therotation stage 10, the illumination spot 20 is scanned in acircumferential direction S2 of a circle having the rotational axis ofthe illumination spot 20 as its center and, due to a translationalmovement made by the translation stage 11, the illumination spot 20 isscanned in a translational direction S1 of the translation stage 11. Theillumination section 101 is configured so that the longitudinaldirection of the illumination spot 20 is parallel to the scanningdirection S1 and, due to scanning in the scanning direction S1, theillumination spot 20 passes through the rotational axis of the rotationstage 10. The movement of the Z stage determines the altitude of thesample 1. That is to say, the movement of the Z stage causes a movementin a direction normal to the surface of the sample 1.

In the above configuration, during one rotation of the sample inscanning in the scanning direction S2, scanning in the scanningdirection S1 is carried out along a distance not longer than thelongitudinal-direction length of the illumination spot 20. Thus, theillumination spot 20 draws a spiral locus T so that the entire surfaceof the sample 1 is scanned.

The detection sections 102 a, 102 b and 102 c are configured to convergescattered light generated at azimuths different from each other and atelevation angles different from each other in order to detect the light.FIGS. 3A to 3D show typical configurations of the detection section 102a.

FIG. 3A shows a typical configuration for a case in which an analogdetection sensor is used. Since configuration elements of the detectionsections 102 b and 102 c are identical with those of the detectionsection 102 a, the configuration elements of the detection sections 102b and 102 c are not explained. It is to be noted that, as will bedescribed later by referring to FIGS. 4A to 4C, in order to detectscattered light in a wide angle range, the layout of the detectionsections is not limited to that of the detection sections 102 a, 102 band 102 c shown in FIG. 1. It is preferable to provide a plurality ofdetection sections having detection directions different from each otherat a number of locations.

As shown in FIG. 3A, the detection section 102 a is configured toarbitrarily make use of a light convergence optical system 81 a, apolarization filter 13 a and a sensor 91 a. The light convergenceoptical system 81 a creates an image of the illumination spot 20 on alight receiving surface of the sensor 91 a or in its vicinity. At theimage creation location, a visual-field diaphragm (not shown in thefigure) having a proper diameter is arbitrarily provided to eliminate orreduce background light generated from locations outside theillumination spot. The polarization filter 13 a can be mounted on anddismounted from the optical axis of the light convergence optical system81 a and can be rotated at a light detection azimuth. The polarizationfilter 13 a is intended to be a filter for reducing scattered lightcomponents caused by typically sample roughness which serves as a noiseorigin.

As the polarization filter 13 a, it is possible to make use of a wiregrid polarization plate or a polarization beam splitter, both having ahigh transmittance and a high extinction factor even for shortwavelengths such as those of ultraviolet light. The wire gridpolarization plate includes one having a structure created by finefabrication of a metallic thin film such as an aluminum or a silver thinfilm on stripes. In order to make it possible to detect very weakforeign scattered light, a photomultiplier tube, an avalanche photodiodeor a semiconductor photo detector coupled with an image intensifier isappropriately used. As the photomultiplier tube for realizing highsensitivity and high precision, it is desirable to make use of aphotomultiplier tube of an ultra bi-alkali type or a super bi-alkalitype. These types have a high quantization factor.

FIG. 3B shows an example wherein a light convergence optical systemcomprising the detection section 102 a, the detection section 102 b andthe detection section 102 c which are shown in FIG. 1 is configured by areflection optical system making use of one elliptical surface mirror701. The light convergence optical system 701 is an elliptical surfacemirror in which the first focal position of the ellipse serves as aradiation position of the illumination light whereas the second focalposition of the ellipse is provided on a light receiving surface of asensor 709 b. The light convergence optical system 701 has a high NAincluding a shallow angle with respect to the surface of a wafer 1 andconverges scattered light. Thus, the light convergence optical system701 fits for guiding the light to the sensors 709 a and 709 b. Inaddition, there is provided a detection section comprising the lightconvergence optical system 701 and the sensors 709 a and 709 b. Thedetection section detects upward scattered light. Thus, there isprovided a configuration capable of detecting light scattered in aplurality of directions at the same time.

FIG. 3C shows a typical configuration of a detection section forconverging scattered light from a plurality of directions and creatingan image on an image sensor. In the configuration, convergence imagecreation systems 88 a, 88 b and 88 c create images on image sensors 99a, 99 b and 99 c from light scattered in a plurality of directionshaving different azimuths or different elevation angles.

By detecting scattered light from the surface of the sample 1, to whichillumination light is radiated as an image and processing the image, itis possible to detect a defect generated in a circuit pattern on thesample 1 which can be a semiconductor wafer or a mask. Thus, theconfiguration is effective for scanning the sample on which the circuitpattern has been created. The image sensors 99 a, 99 b and 99 c can belinear array sensors or 2-dimensional array sensors. The linear arraysensors and the 2-dimensional array sensors are CCD or CMOS sensors. Asan alternative, the image sensors 99 a, 99 b and 99 c can also besensitive image sensors made by connecting image intensifiers to thelinear array sensors or the 2-dimensional array sensors. As anotheralternative, the image sensors 99 a, 99 b and 99 c can also be amulti-anode photomultiplier tube.

FIG. 3D shows a typical configuration making use of a reflection opticalsystem based on a Schwarzschild optical system. Among the scatteredlight generated by the illumination spot 20 shown in FIG. 1, the lightreflected by a concave mirror 711 and converged by a convex mirror 712is converged by an image sensor 92. This configuration is appropriatefor a case in which illumination light having a short wavelength notexceeding 200 nm is radiated to the sample 1 and scattered light fromthe sample 1 creates an image on the image sensor 92.

By referring to FIGS. 4B and 4C, the following description explainsrelations between angular components of scattered light detected by thedetection sections 102 a, 102 b and 102 c. FIG. 4A is an explanatorydiagram to be referred to in description of a display method of adetected angle range. In FIG. 4A, an equatorial plane 401 corresponds tothe surface of the sample 1 whereas the normal direction of the surfaceof the sample 1 is taken as the zenith 402 of a hemisphere. Referencesymbol φ denotes an azimuth angle (longitude) taken the scanningdirection S2 as a reference whereas reference symbol θ denotes an anglefrom the zenith. An angle range detected by the detection sections 102a, 102 b and so on is shown as a region R on the hemisphere. It is shownin FIGS. 4B and 4C and denoted by reference numeral 411. It is obtainedby parallel projection onto a plane 410 parallel to the equatorialplane. The angle ranges detected by the detection sections 102 a, 102 band so on are shown as hatched portions. As is obvious from FIGS. 4B and4C, a plurality of detection sections are provided to cover a wide anglerange. It is thus possible to detect defects having a variety of types.In addition, depending on the defect type and the defect dimensions, theangle distribution of the defect scattered light varies. Thus, by makinguse of a plurality of detection systems to detect the strengths ofscattered light beams having a variety of angles at the same time and bymaking use of a signal processing section to be described later toprocess the signals, it is possible to infer the type of the defect andthe dimensions of the defect with a high degree of precision.

FIG. 4B shows a typical layout of a plurality of detection rangesincluding detection ranges 4901 to 4906 with low elevation angledirections and detection ranges 4911 to 4914 with high elevation angledirections. It shows a typical layout of a detection system fordetecting foreign substances having small to large dimensions. If Ppolarization illumination is carried out, scattered light from a smallforeign substance is radiated strongly in a low angle direction. Bydetecting low angle scattered light components over all azimuths of thedetection ranges 4901 to 4906, a very small defect can be detected. Inaddition, by detecting scattered light components generated at highelevation angles in the detection ranges 4911 to 4914, a dent defect canalso be detected with a high degree of sensitivity. An example of thedent defect is a COP (Crystal Originated Particle) which stronglygenerates scattered light at large angles. On the top of that, byplacing a plurality of detectors in each of the θ and φ directions, itis possible to recognize the characteristic of a scattered lightdistribution which varies in accordance with the defect. FIG. 4C showsthe position of a detection section for carrying out detection at allazimuths in detection ranges 4921 to 4926 in low elevation angledirections. The figure also shows the position of a detection section4931 for detecting scattered light in the direction normal to thesample.

As a light convergence optical system 8, an elliptical surface mirror isused. As shown in FIG. 3B, the elliptical surface mirror makes use of anillumination spot position as one of the focal points. Thus, it ispossible to converge scattered light of all azimuths in a specific θangle range. In addition, by providing spatial filter means or opticalpath branch means on the optical path of the light convergence opticalsystem, it is possible to collectively detect a plurality ofcorresponding detection sections. In either configuration, scatteredlight beams in a wide angle range are caught so that scattered lightbeams with generation directions varying in accordance with defects aredetected. Thus, a variety of defects can be detected robustly. Inaddition, scattered light components in a plurality of directions aredetected individually so that it is possible to classify defects bycomparison with a scattered light distribution library to be describedlater and determine the dimensions of each defect.

The scattered light distribution of a defect depends on the property(index of refraction) of the defect, the shape of the defect and thedimensions of the defect. If the illumination light arrives in aninclined direction, as is commonly known, the larger the lateraldimension of the defect (the defect dimension in the in-plane directionof the sample surface), the more the scattered light slants in theforward direction. In this case, the forward direction is a directionclose to the regular reflection direction of the illumination due to thesurface of the sample. If the lateral dimension of the defect isextremely large in comparison with the wavelength of the illumination(that is, if the lateral dimension is at least 10 times the wavelength),most scattered light components are converged to the vicinity of theregularly reflected light. Thus, to catch a scattered light distributionof a defect with a large lateral dimension, detection of scattered lightscattered in the vicinity of the regular reflection is effective.

FIGS. 5A to 5C show typical configurations of a regularly reflectedlight detection optical system section 104. FIG. 5A shows aconfiguration for optically blocking regularly reflected light from thesurface of a sample 1 by making use of a spatial filter and fordetecting only scattered light very close to the regularly reflectedlight. In this case, illumination light 1011 generated from theillumination section 101 is radiated to the surface of the sample 1. Alens 1041 is provided in such a way that the optical axis of the lens1041 coincides with the optical axis 1040 of regularly reflected lightfrom the sample 1 subjected to radiation of illumination light from theillumination section 101 (refer to FIG. 1) whereas the focal point ofthe lens 1041 coincides with the illumination spot 20. Light coming fromthe illumination spot 20 and passing through the lens 1041 becomesparallel light beams. A light blocking filter 1042 provided on theoptical axis of the lens 1041 optically blocks regularly reflected lightfrom the surface of the sample 1. Light comes from the illumination spot20, slanting with respect to the regularly reflected light. The slantinglight passes through a position separated away from the optical axis bya distance depending on the slanting angle. Thus, only a particularlight component, which has a polarization angle not smaller than anangle corresponding to the size of the light blocking filter, passesthrough the light blocking filter, is converged by a lens 1043, and isdetected by a sensor 1044. In the configuration described above, theregularly reflected light is optically blocked and the strength of ascattered light component close to the regularly reflected light ismeasured. It is to be noted that, by placing a division sensor, such asa four-division sensor, having a plurality of pixels at a position rightbehind the light blocking filter 1042, it is possible to measure thedistribution of the scattered light close to the regularly reflectedlight.

FIG. 5B shows a typical configuration of a detection system based on theSchlieren method. The configuration shown in FIG. 5B can be obtained byreplacing the light blocking filter 1042 of the configuration shown inFIG. 5A with a knife edge 1045. Slight polarization and slight diffusionof regularly reflected light caused by a defect having a size rangingfrom 1/10 times the dimension of the illumination spot to an equivalentsize or an even larger size can be detected as a change in detectionstrength at a sensor 1044.

FIG. 5C shows a typical configuration for carrying out ellipsometryprocessing for light regularly reflected from the surface of the sample.There are a variety of techniques for carrying out the ellipsometryprocessing. In the case of this configuration, however, a phase element1046 and a phase element 1047 are rotated at rotational speeds differentfrom with each other and then the strength of transmitted light isdetected by a sensor 1044. In this configuration, the polarization stateof the regularly reflected light is completely measured so that, on thebasis of a polarization-state change before and after the reflectionoccurring on the sample surface with illumination light, a complexrefraction index of the sample surface and the film thickness can becomputed.

FIGS. 6A to 6C are referred to in explaining the elimination ofvariations caused by a strength distribution of an illumination spot asvariations of the defect scattering strength and explaining acountermeasure against saturation of the signal. In order to converge alight beam generated by a laser light source 2 (refer to FIG. 1) with ahigh degree of efficiency and create a small illumination spot on thesurface of the sample, as the laser light source 2, a light sourceessentially generating a Gauss beam is used. Thus, an illuminationstrength distribution at the illumination spot 20 on the surface of thesample 1 is a Gauss distribution (Refer to FIG. 6A). If the scanningdistance in the S1 direction per scanning rotation made in the S2direction is shorter than the length of the illumination spot in the S1direction, as shown in FIG. 6B, the illumination spot 20 is scanned toresult in a state of overlapping in the S1 direction. At that time, thesame defect is scanned a plurality of times with the defect positionrelative to the illumination spot 20 being changed. Thus, the signal ofthe same defect is detected a plurality of times. If the signal isplotted with the S1 direction taken as the direction of the horizontalaxis, a Gauss distribution identical with the illumination strengthdistribution is drawn. Also with regard to the S2 direction, the signalis sampled in S2-direction scanning for a period shorter than a periodduring which the illumination spot is passing through a defect. Thus, bythe same token, a signal detected a plurality of times from the samedefect draws a Gauss distribution identical with the illuminationstrength distribution in the S2 direction.

FIG. 6C shows a typical signal for a case in which the signal of thesame defect has been detected a plurality of times. A point denoted byan X mark is an actually obtained signal. This graph is a saturatedexample because the signal obtained when the defect passes through thecenter portion of the Gauss distribution, that is, the center portion ofthe illumination strength distribution, exceeds the saturation level ofthe detector. In addition, even if no saturation occurs, the detectionsignal of the defect has variations dependent on a relative positionpassed through by the defect in the illumination spot scanning. In sucha case, since the original Gauss distribution (equivalent to theillumination strength distribution) is known, a true defect signal (adotted line shown in FIG. 6C) can be recovered from a plurality ofobtained signals. By adoption of such a method, it is possible to reducedefect signal variations caused by an illumination strength distributionand effects of the signal saturation. It is to be noted that theillumination strength distribution is not necessarily limited to a Gaussdistribution. That is to say, a homogenizer or the like can also be usedto create an all but uniform illumination strength distribution.

Next, a method for measuring spatial broadening of a defect with a highdegree of precision is explained by referring to FIGS. 6D and 6E. Sincethe scanning speed is assured, the size of the illumination spot isincreased to a large value of the order of several tens of microns. Onthe other hand, the defect is regarded as a point having no area. Asdescribed earlier, however, in the case of a defect having a lateraldimension of at least 10 times the wavelength (a lateral dimension of atleast several microns), since the scattered light is converged to analmost regular reflection direction vicinity, information obtained froma scattered light distribution is little so that classification may bedifficult. For such classification and such a dimension measurement,utilization of particular information, which indicates the number ofsampling operations to detect the signal in scanning, is effective. Asshown in FIG. 6D, however, the profile of the detection signal has ashape obtained by convolution of a instrumental function into theoriginal signal (spatial broadening of the defect). Thus, theinstrumental function limits the resolution of the measurement of thedefect spatial broadening. In order to solve this problem, as shown inFIG. 6E, the profile (a dotted line shown in FIG. 6C) obtained bydeconvolution carried out on the profile of the detection signal bymaking use of the instrumental function is used as an index. Thus, it ispossible to measure the spatial broadening of the defect at a highresolution. In this case, the instrumental function means signalspreading caused by the illumination, detection and processing systems.In the configuration of this device, the instrumental function is equalto the illumination strength distribution. If the response speed of thedetector or the processing system is low for signal sampling, the signalrounding caused by the low speed is also reflected in the instrumentalfunction. The instrumental function is capable of carrying out actualmeasurement by measuring the detection signal profile of a defect havingno spatial broadening (a defect considered to be a point).

FIGS. 7A to 7C show typical configurations of the detection section 102a making use of a photon counting sensor. In this case, as aconfiguration corresponding to that shown in FIG. 3A, configurationsmaking use of a photon counting sensor are shown. However,configurations corresponding to those shown in FIG. 3B to 3D can also beprovided by making the same changes as that shown in FIG. 3A. Theirexplanations are omitted from the following description. Referencenumeral 9001 denotes a photon counting sensor. It is necessary toradiate light as uniformly as possible to this sensor. FIG. 8 shows amore concrete configuration of the photon counting sensor 9001. In thephoton counting sensor 9001, a number of pixels 9101 are created. When aphoton hits any of the pixels 9101, electric charge accumulated inelectric-charge accumulation means (not shown in the figure) employed ineach of the pixels 9101 is discharged to output a pulse signal. Thus,even if 2 or more photons hit 1 pixel at the same time, only one pulseis output. In addition, once a pulse is output from a pixel, no morepulses are output from the pixel as long as no electric charge isaccumulated in the electric-charge accumulation means of the pixelthrough an electric charging operation or, even if another pulse isoutput, the strength of the pulse decreases undesirably. Thus, theconfiguration needs to be set so that photons hit the same pixel asrarely as possible.

Thus, in the configuration shown in FIG. 7A, the photon counting sensor9001 is provided at a location shifted away from the light convergencepoint 81 of a light convergence system 8. In the case of such a layout,however, the light quantity distribution in the photon counting sensor9001 is undesirably a Gauss distribution so that it is difficult to saythat the light is radiated uniformly. Thus, in the configuration shownin FIG. 7B, a non-spherical surface lens 8001 is provided so that lightis radiated to the photon counting sensor 9001 uniformly. Thenon-spherical surface lens 8001 is merely used to make an image uniformin the photon counting sensor 9001. The image is an image of scatteredlight and created on the sample as an image generally having a lineshape. In place of a non-spherical surface lens, it is also possible tomake use of a diffraction optical device.

In addition, in the configuration shown in FIG. 7C, in place of thenon-spherical surface lens 8001, a plurality of prisms 8002 are used.FIG. 7C corresponds to the structure seen from only the horizontaldirection, two prisms are seen. However, four prisms may be used. As analternative, it is also possible to make use of an optical block with ashape having beam optical axes changing at different positions on theoptical path.

By making use of the prisms 8002 light guided to the photon countingsensor 9001 through each of the prisms 8001 is refracted when leavingthe prism 8001 so that the optical axis direction is shifted. Thus, adistribution having a Gauss-beam peak position shifted on the surface ofthe photon counting sensor 9001 is superposed thereon so that the peakstrength decreases. As a result, the uniformity of the light quantity onthe sensor surface is improved. In addition, in place of these, ahomogenizer or the like can be used to form an all but uniformillumination strength distribution.

In addition, the configuration shown in either of FIGS. 7A to 7C alsoincludes a polarization filter 13 for reducing the light quantity on theside of the light convergence system 8 with respect to the lightconvergence point 81 and a mechanism for taking in and out thepolarization filter 13. The polarization filter 13 can be taken in andout in accordance with the light quantity. The polarization filter 13 isused for preventing the sensitivity from deteriorating due to anincrease of a photon count in the photon counting sensor 9001 to bedescribed later.

The following description explain merits in inspection making use of thephoton counting sensor 9001 by comparison with a case in which aphotomultiplier tube is used to serve as a detector.

Eq. (1) expresses the minimum amount of light that can be detected bymaking use of a photomultiplier tube.

ENI=(2e. Id. μ. B)^(1/2) /S(W)   (1)

In the above equation, reference symbol e denotes electric charge of anelectron, reference symbol Id denotes an anode dark current, referencesymbol μ denotes a gain, reference symbol B denotes a cutoff frequencyof a detection signal system and reference symbol S denotes an anoderadiant sensitivity.

As scattered light generated by a defect is becoming weak, the defectcannot be recognized unless the gain μ is increased. If the gain μ isincreased in a photomultiplier tube, however, as is commonly known, thedark-current noises also rise so that the S/N ratio decreases. Thus, inthe case of using a photomultiplier tube, there is a limit to detectionof a small light quantity at a high speed. In addition, there is also aproblem that the photomultiplier tube is relatively weak in withstandingmagnetic noises.

In the case of using a photon counting sensor, on the other hand, themultiplication factor of the light is determined by the amount ofelectric charge accumulated in electric-charge accumulation meansemployed in each pixel. Thus, the multiplication factor can be raisedindependently of the energy of the incident photon. Accordingly, in thephoton counting type sensor 9001, the dark-current noises by no meansincrease even if the signal becomes weak. In the case of a photoncounting sensor, however, if the area of the sensor is increased, thedark-current noises also rise as well. It indicates that, if the numberof pixels increases, the dark-current noises also rise as well. That isto say, the dynamic range and the dark-current noises are in a trade-offrelation.

Next, the inspection mode of the defect inspection device is explainedas follows.

In a defect inspection device, the inspection time and the inspectionsensitivity are generally in a trade-off relation. Thus, in many cases,at a production start-up of samples, highly sensitive inspection with along inspection time is carried out in order to identify problems of themanufacturing process. As the manufacturing process is established and atransition to mass production is made, the inspection time is shortenedand relatively low sensitivity inspection is carried out. In this defectinspection device, the illumination spot 20 is lengthened in thelongitudinal direction, the area which is inspected while the sample 1is being rotated once is increased and the inspection time is shortened.

In order to improve the sensitivity, it is necessary to increase theamount of scattered light from the defect as much as possible. In orderto prevent the sample from being damaged and the characteristic of thedefect from changing, however, the amount of the light is restricted.The amount of the light is set so that the light quantity per unit areais approximately fixed. Thus, if the illumination spot 20 is lengthenedin the longitudinal direction, the illumination light quantity can beincreased so that the amount of scattered light from the sample 1serving as an object of inspection is also raised as well. Since theamount of scattered light from the sample serving as an object ofinspection is different from the defect signal, the scattered light fromthe sample serving as an object of inspection is called backgroundscattered light.

As a result, as the throughput is increased, the amount of thebackground scattered light detected by the photon counting sensor 9001also rises so that many photons of the background scattered light outputpulses from pixels. Thus, even if the defect scattered light is incidentto a pixel of the photon counting sensor 9001, the probability that apulse cannot be output rises, lowering the strength of the signal. Inorder to apply the photon counting sensor 9001 to such a defectinspection device, it is necessary to make use of the photon countingsensor 9001 by reducing the background scattered light. Thus, apolarization filter 13 is used to suppress the background scatteredlight. In order for the polarization filter 13 to suppress thebackground scattered light effectively, it needs for the polarizationdirection of the defect scattered light being different from thepolarization direction of the background scattered light. If thepolarization direction of the defect scattered light is the same as thepolarization direction of the background scattered light, very littledefect scattered light is also suppressed at the same time along withthe background scattered light. Thus, the sensitivity adverselydeteriorates.

In order to increase the amount of scattered light from a small defecton the sample, it is desirable that the illumination light is incidentat polarization P. At that time, scattered light from a defectsufficiently small for the wavelength is scattered mainly at a smallelevation angle and the polarization direction vibrates in the normaldirection of the sample.

FIG. 9 shows the polarization direction of scattered light at everyazimuth. The scattered light is light which is scattered from a sample 1in directions shown in an rθ coordinate system when illumination lighthits the upper surface of the sample 1 in a direction 2900. The centerof a circle 91 shown in FIG. 9 shows the normal direction of thescattering light from the sample 1. The distance r from the center ofthe circle 91 corresponds to an angle (an elevation angle) from thedirection normal to the sample 1 whereas the angle θ corresponds to anangle (an azimuth angle) from the incidence direction of theillumination. Reference numeral 2901 denotes front scattered light,reference numeral 2902 denotes side scattered light whereas referencenumeral 2903 denotes back scattered light.

The polarization direction of light scattered from a defect is adirection almost normal to the sample. Thus, in the front scatteredlight 2901, the polarization direction of light scattered from a defectis perpendicular to the polarization direction of light scattered fromthe sample so that only the background scattered light can be eliminatedby making use of the polarization filter 13. In the back scattered light2903, however, the polarization directions of the defect scattered lightand the background scattered light almost coincide with each other.Thus, the background scattered light 2903 cannot be eliminated by makinguse of the polarization filter 13. In addition, in the side scatteredlight 2902, an intermediate characteristic between these two above. Onthe top of that, besides what is described above, if the sample 1 is asilicon wafer for example, due to its polarization characteristic, aphenomenon occurs as follows: the background scattered light is strongin comparison with the front scattered light.

At that time, the polarization filter 13 can be used. Thus, thebackground scattered light can be made less strong in comparison withthe front scattered light 2901 by about 10% to 20%. As a result, in anarea with little background scattered light, a defect can be detected.At locations of the side scattered light 2902 and the back scatteredlight 2903, on the other hand, it is necessary to detect a defect undera condition with much background scattered light.

If the background scattered light quantity is large, rather thandark-current noises advantageous to the photon counting sensor 9001,shot noises caused by the background scattered light serve as a maincause. Thus, instead of reducing the dark-current noises, the shotnoises need to be decreased. That is to say, it is important to improvethe photon detection efficiency.

FIG. 10 shows detector outputs in the vertical axis for light-quantityinputs in the horizontal axis. Reference numeral 21001 denotes an outputcharacteristic of a photomultiplier tube whereas reference numeral 21002and 21003 each denote an output characteristic of a photon counting typephotosensor. In comparison with the photon counting type photosensorhaving the output characteristic 21002, the photon counting typephotosensor having the output characteristic 21003 is created from manypixels. The photon counting type photosensor having the outputcharacteristic 21003 is capable of obtaining better S/N ratio for smallincident light quantities. Since the size of the pixel is large, theratio of an insensitive area surrounding a pixel to the pixel is small.It can thus be expected that photons are detected with a high degree ofefficiency. On the other hand, the photon counting type photosensorhaving the output characteristic 21002 is created from a number ofpixels. Thus, the ratio of an insensitive area surrounding a pixel tothe pixel is large. As a result, the shot noise is large and thesensitivity is poor. If the gain is increased in the photon countingtype photosensor having the output characteristic 21001, thedark-current noise increases. Thus, the gain cannot be increased. Inaddition, besides the shot noise and the dark-current noise, a magneticnoise generated by a motor or the like is detected with ease in thedefect inspection device. The magnetic noise reduces the sensitivity.

This situation changes in case the incident light quantity increases. Inthe case of the output characteristic 21001, even if the incident lightquantity increases, the relation between the output and the incidentlight quantity is linear. In the case of the output characteristic 21002and the output characteristic 21003, however, the probability that aplurality of photons hit a pixel increases so that, even if the incidentlight quantity increases, the output does not rise that much. Thisbrings about the same effect as reducing photon detection efficiency andcauses reducing the S/N ratio in detecting defects. The figure showscharacteristics in which, in the first place, the output characteristic21003 decreases and, then, the output characteristic 21002 decreases. Inan area with a large light quantity, the output characteristic 21001 isbest.

Due to such characteristics, for the photomultiplier tube, theperformance of the photon counting type photosensor changes much inaccordance with whether the background scattered light is strong orweak. In the case of weak background scattered light, due to theaperture ratio and the high gain, the performance of the photon countingtype photosensor with relatively few pixels is good. As the backgroundscattered light increases, the performance of the photon counting typephotosensor with relatively large number of pixels and having aperformance of the photon counting type photosensor with a low gain anda low aperture ratio is good. In addition, if the background scatteredlight increases further more, the performance of the photomultipliertube is good. Thus, in a defect inspection device for inspecting sampleshaving a variety of types by making use of different illumination lightquantities, in order to increase the sensitivity, it is important toprovide photon counting type photosensor having different pixel countsand a photomultiplier tube as a combination.

In a case the background light quantity is small, when the scatteredlight quantity from the defect itself increases so that the detectionefficiency of the photon counting type photosensor decreases and causingdeterioration of the S/N ratio, even if it happens, the reduction of theS/N ratio does not much affect the defect detection. This is becausedefect scattered light with a sufficient quantity has already beenincident in the photon counting type photosensor. A problem is raised ifthe amount of background scattered light increases. The amount of thebackground scattered light can be reduced by making use of apolarization filter 13 at the position of the front scattered light2901. Thus, a photon counting type photosensor having the characteristic21003 can be used there. At the position of the side scattered light2902, on the other hand, as a whole, the sensitivity can be improved bymaking use of a photosensor having the output characteristic 21003 orthe output characteristic 21002. By the same token, at the position ofthe back scattered light 2903, as a whole, the sensitivity can beimproved by making use of a photosensor having the output characteristic21002 or the output characteristic 21001.

Next, defect dimension estimation and defect classification areexplained. In order to estimate the dimension of a defect with a highdegree of accuracy, it is necessary to accurately find the defectscattered light quantity obtained in every photosensor. If a photosensorundesirably gets saturated, however, it is no longer possible to findthe defect scattered light quantity with a high degree of accuracy.

In order to solve this problem, a configuration like one shown in FIG.11 is adopted. The configuration shown in FIG. 11 is used to prevent thephotosensor from saturation in the configuration shown in FIG. 7A fromgetting saturated. Reference numerals 9002 and 9003 each denote a photoncounting type photosensor. Reference numeral 22001 denotes a beamsplitter for splitting a beam unequally. For example, the beam splitter22001 splits an input beam into output beams propagating to the photoncounting type photosensor 9002 and the photon counting type photosensor9003 at a ratio of 1:16. In this case, the photon counting typephotosensor 9002 is saturated at a relatively small light quantity butthe photon counting type photosensor 9003 is not saturated. Thus, anaccurate light quantity can be found. As an alternative, it is possibleto adopt a configuration employing a photomultiplier tube in place ofthe photon counting type photosensor 9003 in order to avoid saturation.

Even if the photon counting type photosensor 9003 is not saturated, asshown in FIG. 10, nonlinear output characteristics exist. It is thusnecessary to compute a scattered light quantity on the basis of thenonlinear output characteristic. In order to estimate a scattered lightquantity, the signal processing section 105 is provided with a table forcomputing a scattered light quantity from a signal output from thephotosensor. In place of such a table, an approximation formula can alsobe used. Let reference symbol f denote an equation for inferring ascattered light quantity from this output value. In this case, thedefect scattered light quantity is expressed by Eq. 2 given as follows.

$\begin{matrix}\begin{matrix}{I = {{\sum\limits_{i}\; {f_{i}(x)}} - \overset{\_}{\sum\limits_{i}\; {f_{i}(x)}}}} \\{\approx {{\sum\limits_{i}\; {f_{i}(x)}} - {\sum\limits_{i}\; {f_{i}\overset{\_}{(x)}}}}}\end{matrix} & (2)\end{matrix}$

In the above equation,

-   X: Quantity of light detected by a photon counting type photosensor-   X: Time average of quantities of light detected by a photon counting    type photosensor

As described above, a defect signal obtained in each photosensor and atime average of such defect signals are taken as background scatteredlight quantities. The time average is found over a sufficiently longperiod from the sum of background scattered light signals. By making useof the equation f, the defect signal obtained in each photosensor andthe background scattered light are transformed into a linear space inorder to find a scattered light quantity.

Next, a defect determination section 51 composing the signal processingsection 105 is explained by referring to FIG. 12. The followingdescription explains processing for a case in which 2 light detectionsystems are used. Output signals 501 and 502 from an analog processingsection 50 are supplied to high-pass filters 604 a and 604 brespectively to generate respective defect signals 603 a and 603 b whichare supplied to a defect determination section 605. Since the defect isscanned by the illumination spot 20 in the S1 direction, the waveform ofthe defect signal enlarges or shrinks an S1-direction illuminationprofile of the illumination spot 20. Thus, the high-pass filters 604 aand 604 b each pass a frequency band including the defect signalwaveform, blocking a frequency band, which includes relatively numerousnoises, and the direct-current component. As a result, the S/N ratios ofthe defect signals 6041 a and 6041 b are improved.

Each of the high-pass filters 604 a and 604 b is a high-pass filterdesigned to have a specific cutoff frequency and block components havingfrequencies higher than the cutoff frequency. As an alternative, each ofthe high-pass filters 604 a and 604 b can also be a band-pass filter. Asanother alternative, each of the high-pass filters 604 a and 604 b canalso be a filter for forming a figure similar to the waveform of adefect signal in which the shape of the illumination spot 20 isreflected. The defect determination section 605 carries outthreshold-value processing on the input signals 6041 a and 6041 bincluding defect waveforms output by the high-pass filters 604 a and 604b respectively in order to determine whether or not a defect exists.That is to say, since the defect determination section 605 receivesdefect signals based on detection signals from a plurality of lightdetection optical systems, the defect determination section 605 cancarry out threshold-value processing on the sum of a plurality of defectsignals or a weighted average of the defect signals. As an alternative,the defect determination section 605 can also carry out typically OR orAND processing on a group of defects in the same coordinate system seton the surface of a wafer and performs defect inspection with a veryhigh degree of sensitivity comparing to the result of the OR or ANDprocessing with defect detection based on a single defect signal.

Then, for a position at which a defect has been determined to exist, thedefect determination section 605 outputs defect coordinates asinformation on the defect. The defect coordinates are a defect positionin the wafer. The defect position is computed on the basis of a defectwaveform and a sensitivity information signal.

As described above, the analog processing section 50 generates theoutput signals 501 and 502. The output signal 501 is supplied to thehigh-pass filter 604 a and a low-pass filter 601 a whereas the outputsignal 502 is supplied to the high-pass filter 604 b and a low-passfilter 601 b. The high-pass filter 604 a, the low-pass filter 601 a, thehigh-pass filter 604 b and the low-pass filter 601 b are employed in thedefect determination section 51. The low-pass filters 601 a and 601 beach output low-frequency components and the direct-current component.The low-frequency components and the direct-current component arecomponents of a scattered light quantity (haze) from minute roughness inthe illumination spot 20 on the wafer 1. The low-pass filters 601 a and601 b supply outputs 6011 a and 6011 b respectively to a haze processingsection 606 for carrying out haze information processing. That is tosay, the haze processing section 606 outputs a haze signal indicatingthe magnitude of a haze for each location on the wafer 1. The hazesignal is a signal computed from the magnitudes of the input signals6011 a and 6011 b received from the low-pass filters 601 a and 601 brespectively. In addition, in accordance with a spatial frequencydistribution of minute roughness, an angle distribution of lightscattered from the roughness changes. Thus, as shown in FIG. 1, hazesignals are used as inputs to the haze processing section 606 fromdetectors of a plurality of light detection sections 102 provided atlocations with azimuths different from each other and elevation anglesalso different from each other. By making use of haze signals as inputsto the haze processing section 606, it is possible to obtain informationon the spatial frequency distribution of little roughness from, amongothers, strength ratios computed by the haze processing section 606.

Reference numeral 607 denotes a storage section used for temporarilystoring outputs of all detectors, such as the outputs 6041 a and 6041 bof the high-pass filters 604 a and 604 b respectively as well as theoutputs 6011 a and 6011 b of the low-pass filters 601 a and 601 brespectively. Reference numeral 608 denotes a storage section forstoring information on a defect data determined as a defect in thedefect determination section 605. By storing only the defect data andthe quantity of scattered light from the sample 1, it is possible toprevent the size of the storage section 608 from increasing.

Reference numeral 52 denotes adefect-size-inference/defect-classification section for correctingnonlinearity of the output from the photon counting sensor 9001 in orderto estimate a correct defect size and to classify defects. An estimatedsize and a defect classification result are supplied to an entiretycontrol section 53 to be output to a display section 54 or the like.

The defect-size-inference/defect-classification section 52 computesdefect coordinates on the basis of the center of gravity of a defectwaveform. A defect dimension is calculated on the basis of anintegration value of the defect waveform or its maximum value. If asaturated signal magnitude or a signal magnitude close to a saturationvalue has been obtained in a specific detector, the quantity ofscattered light is computed on the basis of outputs of detectors otherthan the specific detector which has been saturated.

The following description explains a system for computing a detectedscattered light quantity from detection signals of detectors. FIG. 13 isa model diagram showing a photon counting sensor 9001 for detectingscattered light. Reference numeral 2301 denotes a strength distributionof scattered light.

The strength distribution of scattered light from the sample and thedistribution of scattered light from a defect do not necessarily providea state in which strengths are uniform. In such a state, scattered lighthits the photon counting sensor 9001. The distribution of this scatteredlight may change in some cases in accordance with the type of the sampleand the polarization condition of the illumination. Desirably, it isnecessary to find a way to compute the brightness from the output of thedetector for every scanning condition. Thus, the following informationis stored. The information includes outputs which are obtained whenevery photon counting sensor 9001 detects scattered light from thesurface of the sample or a foreign substance. In this case, the detectedlight is light scattered when the illumination strength is changed bythe attenuator 3.

FIG. 14 shows a sample 1 for calibration. As shown in the figure,standard samples 31 to 34, whose particle diameter sizes are known, aredistributed in advance on the sample 1. When illumination light withpolarization P is irradiated to the sample 1, it is known that thescattered light quantity is proportional to the 6th power of theparticle diameter. In a case that a particle diameter size issufficiently small in comparison with a wavelength of the illuminationlight, the radiation direction of the scattered light is all butisotropic.

This sample is used for obtaining different incident light quantities.When the incident light quantity is changed, the photon counting typephotosensor detects scattered light from the sample and the standardparticles, generating outputs which are shown in FIG. 15. Referencenumeral 2500 denotes an output signal which is generated by the photoncounting type photosensor 9001 when the photon counting type photosensor9001 detects scattered light from the surface of the sample. Outputsignals in an area enclosed by a line 2501 are outputs which aregenerated by the photon counting type photosensor 9001 when the photoncounting type photosensor 9001 detects scattered light from a particulararea on the sample 1. The particular area is an area in which thestandard particles 31 to 34 whose particle diameter sizes are known havebeen distributed in advance. If the incident light quantity of theillumination is small, the output signal 2500 generated by the photoncounting sensor 9001 is considered to be about equal to an actual photonincidence quantity. Thus, an incident light quantity 2502 is obvious.However, gaps representing incident light quantities 2503 to 2506 arenot necessarily obvious. Since the scattered light quantity isproportional to the 6th power of the particle diameter, however, ratiosof the gaps representing incident light quantities 2503 to 2506 areknown.

An output Oi generated by the photon counting sensor 9001 is ideallyexpressed by Eq. 3 given as follows:

O _(i) =A _(i)(1−P _(i) ^(Ni))   (3)

Eq. 3 is changed to Eq. 4 given as follows:

N _(i)log P _(i)=log(A _(i) −O _(i))   (4)

Unknown variables in the above equation are explained as follows.Reference symbol N_(i) denotes a quantity of incident light hitting aphoton counting type photosensor i. The light hitting the photoncounting type photosensor i corresponds to output signals of an areaenclosed by the line 2501. Reference symbol P_(i) denotes a probabilitythat photons are incident to pixels other than a certain specific pixel.Reference symbol log P_(i) denotes the logarithmic value of theprobability P_(i). If the incident light quantity N_(i) is multiplied byK, in Eq. 4, an equation obtained by different incident light quantitiesN_(i) is derived. Thus, log P_(i) can be found from simultaneousequations. Since it is not necessarily the output ideally expressed byEq. 3, however, measurement making use of an illumination strength iscarried out a plurality of times. Then, parameters are found by adoptionof the least square method or the like in order to compute a correctedcurve.

This sequence is explained by referring to FIG. 16. First of all, at astep S2601, an illumination strength is determined. Then, at a stepS2602, the sample 1 is inspected and a signal output by every photoncounting sensor 9001 detecting scattered light from the surface of thesample 1 and the standard particles is stored. Then, at a step S2603,representative scattered light strengths of the surface of the sample 1or the standard particles are computed. The representative scatteredlight strengths include typically an average value, a median value, or apeak value, which are each a value computed from the scattered lightstrengths of the surface of the sample 1 and the standard particles.

Then, the sequence goes on to a step S2604 to determine whether or notthe processing described above has been carried out a predeterminednumber of times. If the processing described above has not been carriedout the predetermined number of times, the sequence goes back to thestep S2601 to change the illumination strength and inspect the sample 1again in order to obtain an output signal of each photon counting sensor9001. If the processing to obtain data has been carried out thepredetermined number of times, on the other hand, the sequence goes onto a step S2605 at which the least square method is adopted to computethe unknown variable log P and estimate an incident light quantity onthe basis of the variable log P. The incident photon quantity is thequantity of photon hitting every photon counting sensor 9001 from thesurface of the sample 1 and the standard particles for each illuminationstrength.

Then, at a step S2606, the incident light quantity and the output of thephoton counting type photosensor (light detector) 9001 are computed bymaking use of a conversion formula expressed by Eq. 3. In this case,since log P actually varies in accordance with the incident lightquantity (In actuality, an ideal curve is not obtained due to, amongothers, differences in scattered strength distribution inside the lens),for each quantity, the value of P is newly set by making use of itsclose light quantity data in interpolation so that the value of Pcorresponds to actual data. Then, the sequence goes on to a step S2607to create a table for converting the output of each photon countingsensor 9001 into an incident photon count.

As described above, the quantity of light incident to every photoncounting sensor 9001 can be found. However, the estimated precision ofthe incident light quantity obtained by each photon counting sensor 9001varies. That is to say, if the output of the photon counting sensor 9001also substantially changes when the detected light quantity changes,high estimation precision of the incident light quantity is required.Processing carried out by the defect determination section 605 isexplained as follows.

In the photon counting sensor 9001, the first deviation of the output ofthe photon counting sensor 9001 changes for a case in which thebrightness changes due to the incident light quantity. Thus, the photoncounting sensor 9001 for finding the incident light quantity changeswith the light quantity. For this reason, estimation of the defect sizeis explained.

If noises are assumed to be quantization noises when the sum of signalsoutput by all the photon counting type photosensors 9001 is computed,the sum can be computed by multiplying the signals output by the photoncounting type photosensors 9001 by weights which are proportional tosignal strengths detected by the photon counting type photosensors 9001but inversely proportional to background scattered light. In this way,the ratio of the defect signal to noises can be maximized. This isexpressed by Eq. 5 given as follows.

$\begin{matrix}\begin{matrix}{O_{all} = {{\sum\limits_{i\mspace{11mu}}^{\;}\; {g_{i}O_{i}}} = {{\sum\limits_{i\mspace{11mu}}^{\;}\; {g_{i}\left( {{\overset{\_}{O}}_{i} + \Delta_{i}} \right)}} = {\sum\limits_{i\mspace{11mu}}^{\;}\; {g_{i}\left( {{\overset{\_}{O}}_{i} + s_{i} + n_{i}} \right)}}}}} \\{= {\sum\limits_{i\mspace{11mu}}^{\;}\; {g_{i}\left( {{\overset{\_}{O}}_{i} + {\omega_{i}{Is}_{i}} + {\omega_{i}{In}_{i}}} \right)}}}\end{matrix} & (5)\end{matrix}$

With regard to the nonlinearity of the photon counting sensor 9001, anactual aperture ratio of the detector i can be regarded to havedecreased to g_(i) times a reference aperture ratio of the detector i.In this case, the actual aperture ratio is an aperture ratio for a casein which background scattered light increases and a photon hits a pixelalready generating an output. On the other hand, the reference apertureratio is an aperture ratio for a case in which the detected lightquantity is 0.

In Eq. 5, reference symbol s₁ denotes the strength of a defect scatteredsignal whereas reference symbol n_(i) denotes a noise scattered signal.Both are frequency components. Reference symbol ω_(i) corresponds to aderivative value of the output of the photosensor for light which isincident when the light quantity to be detected is 0. The derivativevalue ω_(i) is expressed by a gradient at an incident light quantity of0 on a graph shown in FIG. 15. The graph represents a relation for theincident light quantity of the photon counting type photosensor.Reference symbols ls_(i) and ln_(i) denote a defect scattered lightsignal component and a noise component, respectively.

Reference symbol O_(all) denotes a sum obtained by integration ofsignals output by the photosensors. From Eq. 5, it is obvious that thesum O_(all) is regarded as a sum obtained by multiplying each of thesignals by a gain g_(i)ω_(i). g_(i) changes in accordance with thequantity of light incident to the photosensor. In this case, however,the signals are integrated to result in the sum O_(all.) Thus, by makinguse of the sum O_(all) only, gi cannot be determined so that it isdifficult to find the particle diameter of the defect. In order to solvethis problem, after a defect has been determined, it is necessary tostore signals output by the photosensors so that the photosensor signalswhich have been used in the computation of the sum O_(all) can bereferred to. In order to implement this configuration, storage sections607 and 608 are used. After a defect has been determined, thephotosensor signals stored in the storage sections 607 and 608 arereferred to in order to compute the quantity of light incident to eachphotosensor by making use the formula expressed by Eq. 2. Thus, a defectdimension can be computed.

A flowchart of the processing described above is shown in FIG. 17.

First of all, at a step S2701, for scattered light of a position atwhich a defect has been determined, a signal passing through a high-passfilter is read out from the storage section 608. Then, at a step S2702,for scattered light of the position at which a defect has beendetermined, a signal passing through a low-pass filter is read out fromthe storage section 608. Then, at a step S2703, the output of the photoncounting type photosensor at the position at which a defect has beendetermined is found from these 2 signals. Then, the quantity of lightreceived by the photosensor is computed by making use of the tablecomputed at the step S2607. Then, at a step S2704, the background outputof the photon counting type photosensor at the position at which adefect has been determined is found. Then, the quantity of lightreceived by the background of the photosensor is computed also by makinguse of the table computed at the step S2607. From the difference betweenthe light quantities found at the steps S2703 and S2704, the quantity oflight received by the photosensor from the defect is found. Then, a stepS2706 is executed to set a repetition processing control sectionemployed in the defect inspection device to control all the photosensorsto carry out the processing from the step S2701 to the step S2705 orcontrol photosensors sufficiently received scattered light to carry outthe processing from the step S2701 to the step S2705.

Then, the flow of processing goes on to a step S2707 to find the sum ofdefect received-light quantities computed by the photosensors. Then, ata step S2708, the dimension of the defect is computed on the basis ofthese received-light quantities. If the dimension of the defect is smallin comparison with the wavelength, the dimension of the defect isestimated on the basis of the fact that the defect scattered lightquantity is proportional to the square of the volume of the defect. Asan alternative, the dimension of the defect can also be estimated on thebasis of a relative ratio to a scattered light quantity detected by acommonly known standard particle.

If this is implemented, however, a big error may be generated incomputation of scattered light by a photosensor having a smallg_(i)ω_(i). Thus, in different embodiments, a model data vector is usedto eliminate the error.

Here, g_(i)ω_(i) is equal to a gradient of the relation shown in FIG. 15as a relation associating the incident light quantity with the photoncounting type photosensor. Thus, g_(i)ω_(i) is computed in advance. Forexample, the scattered light quantity to be found is expressed by Eq. 6given as follows:

S1=[1 1 . . . 1] [N₁N₂ . . . N_(n)]^(T)   (6)

Assume that the sum of outputs actually generated by the photosensors isexpressed by Eq. 7 given as follows.

S2=[g₁ω₁g₂ω₂ . . . g_(n)ω_(n)] [N₁N₂ . . . N_(n)]^(T)   (7)

In this case, to find outputs of Eq. 6 from detection results of Eq. 7,a model data vector is used.

In order to implement this, in a state in which the light quantity ofthe illumination is small, that is, in a state in which the photoncounting type photosensor has not been saturated, the quantity of lightincident to the photosensor is computed in advance from the obtainedoutput of the photosensor. Then, vectors [N₁N₂ . . . N_(n)] shown in Eq.7 are computed to be used as a model data vector M_(n).

Here, an output S2/S1 varying due to the transformation from Eq. 6 toEq. 7 is expressed by Eq. 8 given as follows.

S2/S1=[g ₁ω₁ g ₂ω₂ . . . g _(n)ω_(n) ]M _(n) ^(T)/[1 1 . . . 1]M _(n)^(T)   (8)

By multiplying this reciprocal, it is possible to obtain an output witha corrected difference in sensitivity among photosensors.

When the photon counting type photosensor starts getting saturated, thevalue of Eq. 8 changes. By computing the reciprocal of this value, it ispossible to correct a difference in sensitivity between photosensors.

As above, without actually computing the detected light sensitivity ofthis sensor by directly converting the output of the photon countingtype photosensor, it is possible to compute the scattered light quantityto be determined by converting the output on the basis of a model datavector. The flowchart is shown in FIG. 18. Processing carried out atsteps S2801, S2802, S2804 and S2808 is not explained here since that isthe same as the processing carried out at steps S2701, S2702, S2706 andS2708 respectively. Only processing unique to the flowchart shown inFIG. 18 is explained.

In the processing shown in FIG. 17, the quantity of light received by aphotosensor is computed for every photosensor. In the processing shownin FIG. 18, however, g_(i)ω_(i) is computed at a step S2803. Here,g_(i)ω_(i) is a gradient between an incident light quantity of thephoton counting type photosensor and the output of the photosensor. Theincident light quantity is a quantity computed by interpolation at thestep S2606 at an operating point of the output value of eachphotosensor. After this has been repeated number of times equal to thenumber of the photosensors, the value of expression 9 expressingcorrection coefficients is computed at a step S2805.

[g₁ω₁ g₂ω₂ . . . g_(n)ω_(n)]M_(n) ^(T)/[1 1 . . . 1]M_(n) ^(T)   (9)

Then, the sum of output values generated by high-pass filters foroutputs of the photosensor is computed at a step S2806. Then, at a stepS2807, the outputs computed at the step S2806 are each multiplied by acorrection coefficient calculated at the step S2805. The product istaken as the scattered light quantity from a defect from which light isreceived by all the photosensors. Then, at a step S2808, for thisscattered light quantity, the dimension of the defect is computed bycarrying out the same processing as that performed at the step S2708 ofthe flowchart shown in FIG. 17.

In accordance with this embodiment, it is possible to detect both asmall defect generating only weak scattered light and a deformed defectgenerating scattered light only at the rear location.

The present invention innovated by the inventors has been explainedabove in concrete terms on the basis of an embodiment. However, thescope of the present invention is not limited to the above embodiment.Of course, it is possible to make a variety of changes within a rangenot deviating from essentials of the present invention.

REFERENCE SIGNS LIST

1 . . . Wafer,

2 . . . Laser light source,

3 . . . Attenuator,

4 . . . Polarization device,

5 . . . Illumination distribution control device,

6 . . . Convergence lens,

7 . . . Beam expander,

8 . . . Light convergence system,

9 . . . Sensor,

10 . . . Parallel-movement stage,

11 . . . Rotation stage,

13 . . . Polarization filter,

14 . . . Controller,

15 . . . Light blocking shutter,

16 . . . Shutter controller,

20 . . . Illumination spot,

31 . . . Standard particle coating area,

50 . . . Analog processing section,

51 . . . Defect determination section,

52 . . . Defect-size-inference/defect-classification section,

53 . . . Entirety control section,

54 . . . Display section,

55 . . . Input section,

101 . . . Illumination section,

102 a . . . Detection section,

102 b . . . Detection section,

102 c . . . Detection section,

103 . . . Stage section,

104 . . . Regular reflection detection section,

105 . . . Signal processing section.

1-11. (canceled)
 12. A defect inspection device, comprising: anillumination optical unit which illuminates a sample with a laser beamfrom a oblique direction; a detection optical unit which detects lightdiffracted from the sample illuminated with the laser beam by theillumination optical unit; and a signal processing unit which detects adefect on the sample by processing an electrical signal generated bydetecting the light diffracted from the sample with the detectionoptical unit, wherein, the detection optical unit installs pluraldetectors including a photon counting type detector having pluralpixels, the plural detectors including a front detector which detectsforward-scattered light which scattered in forward direction to aillumination direction of the laser beam to the sample by the:illumination optical unit and a backward detector which detectsback-scattered light which scattered in backward direction to aillumination direction of the laser beam to the sample by theillumination optical unit, and a pixel number of the front detector islarger than that of the backward detector.
 13. The defect inspectiondevice according to claim 12, wherein the detection optical unitinstalls a polarizing filter between one of the plural detectors and thesample and cut off a light in a direction perpendicular to a lightscattered from the sample.
 14. The defect inspection device according toclaim 12, wherein the detection optical unit installs a bean splitterbetween one of the plural detectors and the sample and a detector whichdetects a light form the sample and branched by the beam splitter. 15.The defect inspection device according to claim 12, wherein at least oneof the plural detectors is an analog type detector and the frontdetector is a photon counting type detector.
 16. A method of inspectinga defect, including the steps of: radiating a laser beam onto a samplefrom an oblique direction through an illumination optical unit;detecting a scattered light from the sample on which the laser beam isradiated, with a detection optical unit; and detecting a defect on thesample by processing an electrical signal output from the detectionoptical unit by the detection of the scattered light, with a signalprocessing unit, wherein, in the step of detecting the scattered lightfrom the sample, the scattered light is detected by plural detectorsincluding photon counting type detector, and among the plural detectorsa forward-scattered light which scattered in forward direction to aillumination direction of the laser beam to the sample by theillumination optical unit is detected by a front detector andback-scattered light which scattered in backward direction to aillumination direction of the laser beam to the sample by theillumination optical unit is detected by a backward detector, and thefront detector detects the forward-scattered light with sensors a pixelnumber of which is larger than that of the backward detector.
 17. Themethod of inspecting a defect according to claim 16, wherein in the stepof detecting the scattered light with the detection optical unit, thescattered light is detected through a polarizing filter installedbetween one of the plural detectors and the sample and which cut off alight in a direction perpendicular to a light scattered from the sample.18. The method of inspecting a defect according to claim 16, wherein inthe step of detecting the scattered light with the detection opticalunit, one of the forward-scattered light or the back-scattered light isbranched by a beam splitter and detected.
 19. The method of inspecting adefect according to claim 16, wherein at least one of the pluraldetectors of the detection optical system is an analog type detector andthe front detector is a photon counting type detector.